The present invention relates to panels with acoustic attenuation and more particularly to those lining the annular fan channels formed in the nacelles of turbo motors, particularly of aircraft.
Such panels form the envelope, on the fan channel side, of the nacelle, behind the air inlet lip and have a structure adapted to attenuate the noise produced by the central portion of the motor surrounded by the nacelle and particularly the noise produced by the fan.
In practice, these panels include a porous core such as a honeycomb structure covered, on the fan channel side, with an acoustic damping layer and, on the opposite side, a rear reflector.
The acoustic damping layer is a porous structure with the role of dissipating, which is to say partially transforming, the acoustic energy of the sound wave passing through it, into heat.
This porous structure can be, for example, a metallic cloth or a cloth of carbon fibers whose weave permits fulfilling its dissipating function.
These acoustic panels must also have sufficient structural properties particularly to receive and transfer aerodynamic and inertial forces and forces connected with supporting the nacelle, toward the nacelle/motor structural connections, and so it is necessary to impart structural properties to the acoustic damping layer.
To this end, there can, as shown in British patent 2 130 963, provide an acoustic damping layer with two components, namely a structural layer, on the honeycomb side and a porous surface layer, or else use as the damping layer a cloth combining both the acoustic function and the structural function by selecting a diameter of the filaments of the cloth giving to this latter a high resistance to forces coupled with good acoustic damping.
For making such panels, there is known the process consisting in producing the annular assembly forming the wall of the fan channel in two interfitting half channels comprising, for each half panel, the following steps:                predeforming a layer of porous structure on a form identical to a half panel, with the help of jaws stretching the porous material to its elastic limit,        acoustically measuring the shape thus produced so as to qualify the mean value of the acoustic porosity,        adapting to the mean value above, the spacing of winding of the carbon filaments adapted to be deposited on the porous layer to constitute a structural layer,        emplacing on a suitable mold the predeformed shape,        then making the half panel by known techniques of winding said carbon fibers, and emplacing the porous core and the rear reflector.        
This process has drawbacks.
Thus, the shape to be produced not being a figure of revolution, there exists in the deformed layer inhomogeneous regions, which is to say regions stretched and regions stressed, which degrade the general acoustic quality of the porous structure. The interval of winding the carbon fibers being adapted to the mean value of acoustic porosity of the structure, the inhomogeneous regions introduce variations in the acoustic attenuation of the noise generated by the motor.
Moreover, the presence of interfitting connections of the two half panels introduces two regions of acoustic refraction in the final acoustic panel, which is prejudicial to the quality of damping the noises generated by the motor.
To overcome these drawbacks, there can, as taught by French patent 2 767 411 in the name of the applicant, be emplaced an acoustic damping layer by winding with a porous material present in the form of strip.
This manner of proceeding not only permits avoiding inhomogeneous regions of the porous layer from an acoustical standpoint, as indicated above, in the conventional manner of fabricating the two half panels, but also eliminates the necessity of interfitting, the winding of the porous layer being adapted to form other layers, namely the structural layer, the porous central core, the rear reflector, so as to produce a complete acoustic panel in a single piece, without interfitting.
The absence of interfitting permits increasing the effective acoustic surface of the panel, of decreasing its weight and reducing the time and cost of production.
However, an acoustic panel of such a structure uses constituent strips of the porous layer in direct contact with the flow of aerodynamic fluid in the fan channel. When these strips are made of metallic cloth, they are easily peeled back at their edge in contact with the aerodynamic flow, the more so as they are disposed substantially perpendicular to the direction of said flow.
Thus, not only the acoustic qualities of the panel are degraded, but moreover, the panel itself is degraded and must be changed, which gives rise to maintenance and down time costs of the aircraft.
The present invention has for its object to overcome these drawbacks by improving the winding technique described in FR 2 767 411.